1. Field of Invention
The current invention relates to methods of producing graphene and devices and methods of producing the devices using graphene, and more particularly to high-throughput solution processing of graphene and devices and methods of producing the devices using the graphene.
2. Discussion of Related Art
Since its experimental discovery in 2003, there has been a great amount of interest in single layer graphene for a variety of applications. Ballistic transport of electrons along the atomically thin layer, along with mobilities exceeding 15,000 cm2/Vs and an ambipolar field effect make graphene a particularly good candidate for the next round of semiconductor devices (Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.; Electric Field Effect in Atomically Thin Carbon films. Science 2004, 306 (5696), 666-9; Gusynin, V. P.; Sharapov, S. G.; Unconventional Integer Quantum Hall Effect in Graphene, Phys. Rev. Lett. 2005, 95(14), 146801; Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P.; Experimental observation of the quantum Hall effect and Berry's phase in graphene. Nature 2005, 438(7065), 201-204; Novoselov, K. S.; McCann, E.; Morozov, S. V.; Fal'ko, V. I.; Katsnelson, M. I.; Zeitler, U.; Jiang, D.; Schedin, F.; Geim, A. K.; Unconventional quantum Hall effect and Berry's phase of 2pi in bilayer graphene. Nature Physics 2006, 2(3), 177-180; Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Boebinger, G. S.; Kim, P.; Geim, A. K.; Room-Temperature Quantum Hall Effect in Graphene. Science 2007, 315(5817), 1379; Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A.; Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438(7065), 197-200).
Although similarities to carbon nanotubes and other conjugated systems help contribute to the theoretical understanding of graphene, experimental results have been less forthcoming due to the difficulty in producing single layer specimens. As with carbon nanotubes, the large aspect ratio of individual sheets, and strong Van der Waals forces holding them together, make isolating single sheets of graphene very challenging.
Thus far only two methods have enjoyed reliable success; the Scotch tape or “drawing” method and by the reduction of silicon carbide (Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A.; Two-dimensional gas of massless Dirac fermions in graphene. Nature 2005, 438(7065), 197-200). The drawing method utilizes a piece of Scotch tape to draw a thin film from highly oriented pyrolytic graphite (HOPG). After repeated peeling from the thin film, it is ultimately stamped onto a substrate and the tape is carefully removed. The resulting deposition is a dense network of both single and multi-layered graphene, which must be scoured using an optical microscope and otherwise characterized before finally a single sheet may be reliably identified for further use. Alternatively, the reduction of silicon carbide (SiC) reliably produces small regions of graphitized carbon, but requires temperatures greater than 1000° C. (Berger, C.; Song, Z.; Li, T.; Li, X.; Ogbazghi, A. Y.; Feng, R.; Dai, Z.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A.; Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics. J. of Phys. Chem. B 2004, 108(52), 19912-19916; Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A.; Electronic confinement and coherence in patterned epitaxial graphene. Science 2006, 312(5777), 1191-1196).
While these methods have provided adequate samples for preliminary experimental results, both present a number of drawbacks and neither is well suited for industrial applications. First and foremost, the yield of single sheets produced is exceedingly low. Furthermore, the location of those specimens is largely random, and certainly not controllable to the level required for mass fabrication techniques. Finally, neither the peeling method nor the reduction of silicon carbide is scalable or high-throughput. These necessary conditions for the ultimate goal of graphene electronics present formidable hurdles and will continue to motivate research.
Chemists have recently proposed a third synthetic route through the oxidation and exfoliation of HOPG, which may provide a number of advantages (Viculis, L. M.; Mack, J. J.; Kaner, R. B.; A Chemical route to carbon nanoscrolls. Science 2003, 299(5611), 1361; Shioyama, H.; Akita, T.; A new route to carbon nanotubes. Carbon 2003, 41(1), 179-181; Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S.; Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44, 1558-1565; Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S.; Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide Carbon. 2007, 45, 1558-1565; Gomez-Navarro, C.; Weitz, R. T., Bittner, A. M.; Scolari, M.; Mews, A.; Burghrd, M.; Kern, K. Electronic transport properties of individual chemically reduced graphene Oxide Sheets Nano lett. 2007, 7, 3499-3503). The resulting single sheets of oxidized graphite are stable as uniform aqueous dispersions. Although graphite oxide is itself an insulator, the sheets may be restored to semi-metallic graphene, and its planar structure, by chemical reduction or by thermal annealing. The technique has led to a number of functioning single sheet field-effect devices (Gilje, S.; Han, S.; Wang, M. S.; Wang, K. L.; Kaner, R. B.; A chemical route to graphene for device applications. Nano lett. 2007, 7, 3394-3398; Gomez-Navarro, C., Weitz R., Bittner, A. M., Scolari, M., Mews A., Burghard, M, and Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets Nano lett. 2007, 7, 3499-3503). Fabrication typically includes air-brushing or spin-coating from water, followed by an electron-beam process to deposit electrodes, and in situ chemical reduction. Although graphite oxide dispersions facilitate some solution processing, the location of single sheets has been uncontrollable and individual sheets often aggregate due to the high surface tension of water (Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S.; Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44, 1558-1565). In addition, many of the resulting sheets are found to be wrinkled or folded when examined by atomic force microscopy (AFM). Also, cross-sectional step heights of more than 1 nm are often observed for a single sheet which is much larger than the theoretical value of 0.34 nm found in graphite. This increased thickness may be attributed to unreduced surface hydroxyl and epoxide groups. Such functionalities are detrimental to the electrical properties of graphene. Furthermore, aqueous dispersions are not ideal for deposition as the high surface tension of water leads to aggregation during the evaporation process. Finally, even if GO is perfectly deposited, reduction methods tend to neglect the area in direct contact with the substrate. Attempts have been made to complete the reduction stage in solution, but sheets tend to aggregate due to the attractive forces between layers and an overall decrease in hydrophilicity. Therefore, there remains a need for improved methods of producing graphene as well as device made using graphene.